A brushless motor having high efficiency is used as a driving source for a power tool. A motor drive circuit for driving the power tool is provided separately from the brushless motor in a housing. The motor drive circuit is constituted by an inverter circuit having a plurality of switching elements such as a transistor and a field-effect transistor (hereinafter, abbreviated as “FET”).
A high current flows through switching elements, and switching operation is performed at high frequency in the inverter circuit, thereby increasing an amount of heat generated by the switching elements. Therefore, cooling the inverter circuit forcibly is required. To forcibly cool the inverter circuit, a cooling fan to cool switching elements is used as one example. The cooling-air path is defined by forming inlet and exhaust ports on the housing. The switching elements are arranged in the cooling-air path for the forcibly cooling.
Japanese Patent Application Publication No. 2004-015956 discloses that switching elements and a cooling fan are arranged in an axial direction of the motor so that cooling-air is directly sent to the switching elements in order to cool the switching elements more effectively. As a result, the inverter circuit is forcibly cooled down.
Nowadays, downsizing of power tools is highly required in terms of operability in a narrow work space and portability. In particular, downsizing of an entire motor section accommodating the brushless motor and the inverter circuit in the power tool is a subject to be solved.
Power tools are often used in dusty-air environments including iron-powders and wooden powders. Therefore, entry of the dust into the housing should be restricted, which imposes various restrictions on a size and a position of ventilation holes.
The structure in which the switching elements are arranged on the cooling-air path requires securing of larger amount of blowing air. Therefore, a larger-sized cooling fan and larger-sized ventilation holes need to be provided.
However, the securing of a larger amount of blowing air often leads to easy entry of dust such as iron powders and wood powders into the housing, thereby lowering the dust resistance of the power tool. Further a use of the larger-sized fan may increase windage loss of the fan, thereby decreasing motor efficiency.
An object thereof is to provide a power tool which can easily and readily cool a driving circuit therein while preventing entry of dust to a housing and downsizing the power tool.